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Health Sciences
12-2003
Comparison of 3 Methods to Assess Urine Specific
Gravity in Collegiate Wrestlers
Kristin J. Stuempfle
Gettysburg College
Daniel G. Drury
Gettysburg College
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Stuempfle, K., and D. Drury. Comparison of 3 Methods to Assess Urine Specific Gravity in Collegiate Wrestlers. Journal of Athletic
Training (December 2003) 38(4):315-319.
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Comparison of 3 Methods to Assess Urine Specific Gravity in Collegiate
Wrestlers
Abstract
Objective: To investigate the reliability and validity of refractometry, hydrometry, and reagent strips in
assessing urine specific gravity in collegiate wrestlers.
Design and Setting: We assessed the reliability of refractometry, hydrometry, and reagent strips between 2
trials and among 4 testers. The validity of hydrometry and reagent strips was assessed by comparison with
refractometry, the criterion measure for urine specific gravity.
Subjects: Twenty-one National Collegiate Athletic Association Division III collegiate wrestlers provided fresh
urine samples.
Measurements: Four testers measured the specific gravity of each urine sample 6 times: twice by
refractometry, twice by hydrometry, and twice by reagent strips.
Results: Refractometer measurements were consistent between trials (R 5 .998) and among testers;
hydrometer measurements were consistent between trials (R 5 .987) but not among testers; and reagent-strip
measurements were not consistent between trials or among testers. Hydrometer (1.018 6 0.006) and reagentstrip (1.017 6 0.007) measurements were significantly higher than refractometer (1.015 6 0.006)
measurements. Intraclass correlation coefficients were moderate between refractometry and hydrometry (R 5
.869) and low between refractometry and reagent strips (R 5 .573). The hydrometer produced 28% false
positives and 2% false negatives, and reagent strips produced 15% false positives and 9% false negatives.
Conclusions: Only the refractometer should be used to determine urine specific gravity in collegiate wrestlers
during the weight-certification process.
Keywords
refractometer, hydrometer, reagent strips, hydration status
Disciplines
Other Medicine and Health Sciences | Sports Sciences
This article is available at The Cupola: Scholarship at Gettysburg College: http://cupola.gettysburg.edu/healthfac/29
Journal of Athletic Training
2003;38(4):315–319
q by the National Athletic Trainers’ Association, Inc
www.journalofathletictraining.org
Comparison of 3 Methods to Assess Urine
Specific Gravity in Collegiate Wrestlers
Kristin J. Stuempfle; Daniel G. Drury
Gettysburg College, Gettysburg, PA
Kristin J. Stuempfle, PhD, ATC, contributed to conception and design; acquisition and analysis and interpretation of the data;
and drafting, critical revision, and final approval of the article. Daniel G. Drury contributed to analysis and interpretation of the
data and critical revision and final approval of the article.
Address correspondence to Kristin J. Stuempfle, PhD, ATC, Health and Exercise Sciences Department, Campus Box 432,
Gettysburg College, Gettysburg, PA 17325. Address e-mail to [email protected].
Objective: To investigate the reliability and validity of refractometry, hydrometry, and reagent strips in assessing urine specific gravity in collegiate wrestlers.
Design and Setting: We assessed the reliability of refractometry, hydrometry, and reagent strips between 2 trials and
among 4 testers. The validity of hydrometry and reagent strips
was assessed by comparison with refractometry, the criterion
measure for urine specific gravity.
Subjects: Twenty-one National Collegiate Athletic Association Division III collegiate wrestlers provided fresh urine samples.
Measurements: Four testers measured the specific gravity
of each urine sample 6 times: twice by refractometry, twice by
hydrometry, and twice by reagent strips.
Results: Refractometer measurements were consistent between trials (R 5 .998) and among testers; hydrometer mea-
surements were consistent between trials (R 5 .987) but not
among testers; and reagent-strip measurements were not consistent between trials or among testers. Hydrometer (1.018 6
0.006) and reagent-strip (1.017 6 0.007) measurements were
significantly higher than refractometer (1.015 6 0.006) measurements. Intraclass correlation coefficients were moderate
between refractometry and hydrometry (R 5 .869) and low between refractometry and reagent strips (R 5 .573). The hydrometer produced 28% false positives and 2% false negatives,
and reagent strips produced 15% false positives and 9% false
negatives.
Conclusions: Only the refractometer should be used to determine urine specific gravity in collegiate wrestlers during the
weight-certification process.
Key Words: refractometer, hydrometer, reagent strips, hydration status
I
fractometry is a valid indication of hydration status.8–11 In 2
papers assessing urinary indices of hydration status, Armstrong et al8,9 reported that urine specific-gravity measurement
by refractometry was a more sensitive indication of hydration
status than blood measurements, including plasma osmolality,
plasma sodium, or hematocrit. Popowski et al11 also concluded
that measurement of urine specific gravity by refractometry
was a valid assessment of hydration status, although it may
lag somewhat behind plasma osmolality during progressive
acute dehydration. Finally, the recent National Athletic Trainers’ Association position statement on fluid replacement for
athletes stated that urine specific gravity measured by a refractometer should be used to determine the hydration status
of athletes.10
Research to assess the validity of hydrometry and reagent
strips compared with refractometry to determine urine specific
gravity has provided mixed results. McCrossin and Roy12 described the overall correlation between refractometry and hydrometry as ‘‘good.’’ In studies comparing refractometry with
reagent strips, several researchers have suggested that reagent
strips are an acceptable alternative to refractometry,13–15
whereas others have concluded that they are not.12,16–19 Although these findings are contradictory, the interclass Pearson
correlation coefficients reported in these previous validity
studies might not be the most appropriate statistical approach
n 1997, three collegiate wrestlers died while attempting to
reduce weight by dehydration.1–3 To prevent a recurrence
of this tragedy, the National Collegiate Athletic Association (NCAA) introduced new rules in 1998 that discourage
dangerous weight-cutting practices.1–3 These new rules include
a weight-certification process that requires the determination
of hydration status. The NCAA selected urine specific gravity
as the most practical, cost-efficient hydration measure to use
during the weight-certification process.4
Urine specific gravity is a measure of the ratio of the density
of urine to the density of water. Urine specific-gravity measurements normally range from 1.002 to 1.030.5 The NCAA
selected a urine specific-gravity measurement of #1.020 to
indicate euhydration.4 Wrestlers with a urine specific gravity
#1.020 are considered euhydrated and may have their body
composition assessed to determine their minimal weight for
competition, whereas wrestlers with a urine specific gravity
.1.020 are considered to be dehydrated and may not proceed
to body-composition testing on that day.
Refractometry, hydrometry, and reagent strips are commonly used to assess urine specific gravity. In 1998, the NCAA
allowed the use of all 3 methods.4 However, in 1999, the use
of reagent strips was eliminated.6 Previous reports have indicated that refractometry is the criterion measure for urine specific gravity7 and that urine specific gravity measured by re-
Journal of Athletic Training
315
to use when comparing 2 methods of measurement.20,21 Furthermore, none of these authors assessed the reliability of refractometry, hydrometry, or reagent strips.
Therefore, our purposes were to assess the reliability of refractometry, hydrometry, and reagent-strip measurements
across multiple trials and testers and to investigate the validity
of hydrometer and reagent-strip measurements compared with
refractometry using a variety of statistical approaches.
METHODS
Reliability of refractometry, hydrometry, and reagent strips
was assessed between 2 trials and among 4 testers. The validity of hydrometry and reagent strips was assessed by comparison with refractometry, the criterion measure for urine specific gravity.8–11
Subjects
Twenty-one healthy members of an NCAA Division III
wrestling team (age 5 20.0 6 1.29 years, height 5 174.7 6
8.27 cm, mass 5 82.8 6 18.00 kg) each provided one urine
sample. The subjects provided written informed consent, and
the study was approved by the institution’s institutional review
board.
Testers
Three certified athletic trainers and one athletic training student who were experienced in refractometer, hydrometer, and
reagent-strip measurements served as testers.
Instruments
The following instruments were used to assess urine specific
gravity. The Schuco Clinical Refractometer (model 57112021; Williston Park, NY) has a temperature-compensating
dial and graduated intervals of 0.005 units with a scale ranging
from 1.000 to 1.040. It was calibrated with distilled water before use.
Urine samples greater than 60 mL were measured in an
Assistant Urinprober hydrometer (model 242; Sondheim/
Rhon, Germany), which is graduated in intervals of 0.001
units with a scale ranging from 1.000 to 1.060. Urine samples
less than 60 mL were measured in a smaller Assistant Urinprober hydrometer (model 248), which is graduated in intervals of 0.002 units with a scale ranging from 1.000 to 1.060.
Distilled water provided the calibration standard for the hydrometers before use. Hydrometer results were adjusted for
temperature by adding or subtracting 0.001 specific-gravity
units for each 38C above or below 208C, respectively.22
The N-MULTISTIX 10 SG Reagent Strips (Miles Laboratories, Inc, Elkhart, IN) have a specific-gravity scale ranging
from 1.000 to 1.030, with color blocks in intervals of 0.005
units. The reagent strips were placed in the urine and removed
and specific gravity read between 45 and 60 seconds after
removal.
A standard laboratory thermometer measured urine temperature at the time of assessment.
Procedures
Testing was done before the start of the wrestling season,
and subjects had not exercised within 24 hours of testing. Sub-
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• Number 4 • December 2003
jects were not given any special instructions concerning fluid
consumption before testing. Urine samples were collected between 1:00 PM and 3:00 PM Greenwich Mean Time and analyzed immediately. Urine samples were divided into 2 subsamples (trials A and B) for analysis. Each tester measured the
specific gravity of each urine subsample in the following order: reagent strip, hydrometer, and refractometer. All specificgravity measurements were read to the nearest 0.001 specificgravity unit. Therefore, the refractometer, the smaller Assistant
Urinprober hydrometer, and the reagent strips were read to a
precision of less than one marked unit. A recorder wrote down
the specific-gravity measurements on the data sheets so that
testers could not compare the values of the measurements.
Statistical Analysis
We calculated a 3 3 2 3 4 (method 3 trial 3 tester) repeated-measures analysis of variance for significant mean differences in urine specific-gravity measurements among methods, trials, and testers with alpha set at P , .05 and followed
up with Scheffé post hoc comparisons. Intraclass correlation
coefficients were calculated between methods and trials. The
strength of the intraclass correlation coefficients was assessed
according to the rating scale of Vincent.23 We prepared BlandAltman plots21 to evaluate agreement between measurement
methods for urine specific gravity. Refractometer measurements were accepted as the true indication of hydration status.8–11 When the hydrometer or reagent strips indicated results contradictory to the refractometer, the measurements
were recorded as false positives or false negatives. A false
positive was defined as a measurement with the hydrometer
or reagent strip indicating dehydration (value exceeded 1.020),
when the refractometer indicated euhydration (value did not
exceed 1.020). A false negative was defined as a measurement
by the hydrometer or reagent strip indicating euhydration (value did not exceed 1.020), when the refractometer indicated
dehydration (value exceeded 1.020). False positives and false
negatives are reported as a percentage of all samples. The sensitivity of a test indicates how well a test finds disease positives (ie, those with the condition, such as dehydration). The
sensitivity of the hydrometer and reagent strips was calculated
as the number that were both disease positive and test positive,
divided by the number that were disease positive, times 100.24
The specificity of a test indicates how well a test excludes
disease negatives (ie, those without the condition, such as dehydration). The specificity of the hydrometer and reagent strips
was calculated as the number that were both disease negative
and test negative, divided by the number that were disease
negative, times 100.24
RESULTS
The method 3 trial 3 tester interaction was significant
(F6,160 5 4.085, P 5 .0008) (Table 1). The refractometer was
reliable between trials for each tester and among all testers.
The hydrometer was reliable between trials for each tester but
was not consistent among testers. The reagent strips were not
reliable between trials for the testers or among testers.
A significant interaction occurred between method and trial
(F2,160 5 4.079, P 5 .0187) (Table 2). The refractometer and
hydrometer were reliable between trials, but the reagent strips
were not. The tester 3 trial interaction also was significant
(F3,80 5 3.423, P 5 .0211) (Table 2). Tester 1 and tester 2
Table 1. Method 3 Trial 3 Tester Interaction (Mean 6 SD)
Trial A
Trial B
Refractometer
Tester 1
Tester 2
Tester 3
Tester 4
1.015
1.015
1.015
1.015
6
6
6
6
0.006
0.006
0.006
0.006
1.015
1.015
1.015
1.015
6
6
6
6
0.006
0.006
0.006
0.006
Hydrometer
Tester 1
Tester 2
Tester 3
Tester 4
1.018
1.017
1.017
1.018
6
6
6
6
0.006
0.005
0.006
0.006
1.018
1.017
1.017
1.018
6
6
6
6
0.007
0.005
0.007
0.006
Reagent strips
Tester 1
Tester 2
Tester 3
Tester 4
1.018
1.015
1.015
1.018
6
6
6
6
0.007
0.006
0.006
0.006
1.016
1.015
1.019
1.019
6
6
6
6
0.007
0.008
0.007
0.008
Figure 1. Bland-Altman plot for refractometry and hydrometry. The
dotted line represents zero difference between the 2 methods and
the 2 solid lines 62 standard deviations of the difference between
methods.
Table 2. Method 3 Trial and Tester 3 Trial Interactions (Mean 6
SD)
Trial A
Trial B
Method 3 trial interaction
Refractometer
1.015 6 0.006
Hydrometer
1.018 6 0.006
Reagent strips
1.016 6 0.006
1.015 6 0.006
1.018 6 0.006
1.018 6 0.007
Tester 3 trial interaction
Tester 1
Tester 2
Tester 3
Tester 4
1.017
1.016
1.017
1.018
1.017
1.016
1.016
1.017
6
6
6
6
0.006
0.006
0.006
0.006
6
6
6
6
0.006
0.006
0.007
0.007
Table 3. Method and Trial Main Effects (Mean 6 SD)
Method main effect
Refractometer
Hydrometer
Reagent strips
1.015 6 0.006
1.018 6 0.006*
1.017 6 0.007*
Trial main effect
Trial A
Trial B
1.016 6 0.006
1.017 6 0.007†
*Significantly different from refractometer (P , .05).
†Significantly different from trial A (P , .05).
were consistent between trials, but tester 3 and tester 4 were
not.
Main effects were significant for both method and trial (Table 3). Refractometer measurements were significantly lower
than hydrometer or reagent-strip measurements (F2,160 5
8.993, P 5 .0002). Trial A measurements were significantly
lower than trial B measurements (F1,80 5 5.769, P 5 .0186).
Intraclass reliability between trials was high for refractometry (R 5 .998) and hydrometry (R 5 .987) and moderate for
reagent strips (R 5 .854). Intraclass coefficients between refractometry and hydrometry were moderate (R 5 .869) and
low between refractometry and reagent strips (R 5 .573).
Hydrometer measurements were consistently greater than
refractometry measurements (mean difference 5 0.002 6
0.003) (Figure 1). The calculated 95% limits of agreement
indicate that, for 95% of observations, hydrometer values will
Figure 2. Bland-Altman plot for refractometry and reagent strips.
The dotted line represents zero difference between the 2 methods
and the 2 solid lines 62 standard deviations of the difference between methods.
be 0.004 less than or 0.008 greater than refractometer values.
Reagent-strip measurements also tended to be greater than refractometer measurements (mean difference 5 0.002 6 0.007)
with 95% limits of agreement indicating that reagent-strip values are expected to be 0.012 less than or 0.016 greater than
refractometer measurements (Figure 2).
False positives (pass refractometer, fail other method) occurred with both hydrometry (47/168, 28%) and reagent strips
(25/168, 15%). False negatives (fail refractometer, pass other
method) also occurred with the hydrometer (3/168, 2%) and
reagent strips (15/168, 9%). Sensitivity and specificity of the
hydrometer were 88% and 67%, respectively. Sensitivity of
the reagent strips was 38% and specificity was 83%.
DISCUSSION
Two types of questions can be asked in method-comparison
studies. First, what are the characteristics of each method?
How repeatable are the measurements for each method? The
reliability of a method is typically determined by the test-retest
method.23 Second, how do the methods compare? Do the
Journal of Athletic Training
317
Figure 5. Trial-by-tester interaction for reagent strips.
Figure 3. Trial-by-tester interaction for refractometer.
Figure 4. Trial-by-tester interaction for hydrometer.
methods measure the same thing? The validity of a method
may be determined by comparing it with another method
known to be valid.23
To our knowledge, this is the first study to examine both
the reliability and validity of methods commonly used to measure urine specific gravity. Reliability of refractometry, hydrometry, and reagent strips was assessed between trials and
among testers. Validity of the hydrometer and reagent strips
was assessed by comparing them with refractometry, the criterion measure for urine specific gravity.7–11
Examination of the method 3 trial 3 tester interaction (see
Table 1, Figure 3) reveals that the refractometer was reliable
by trial and tester, which was further confirmed by a high
intraclass correlation between trials. This is not surprising because determining the urine specific-gravity value using a refractometer is very objective. Hydrometer readings were consistent between trials for each tester but were not consistent
among testers (see Table 1, Figure 4). This may reflect tester
subjectivity and the difficulty in determining the density-indicating meniscus. Finally, analysis of the data in Table 1 and
Figure 5 reveals that reagent-strip measurements were not consistent between trials for the testers or among testers. This was
expected because a key disadvantage of reagent strips is that
the visual interpretation of color change on the reagent strip
often is difficult and very subjective.
Previous researchers have focused on the validity of hy-
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Volume 38
• Number 4 • December 2003
drometry and reagent strips compared with refractometry,
rather than on the reliability of the 3 methods. McCrossin and
Roy12 reported an overall correlation of .96 between hydrometry and refractometry in 69 urine samples from hospitalized
children. However, the correlation coefficient decreased as the
urine specific gravity increased. Several investigators13–15
have suggested that reagent strips are an acceptable alternative
to refractometry. Gounden and Newall14 compared results using reagent strips with 2 refractometers in 12 normal subjects
and reported correlations of .906 and .911. Guthrie et al15 collected urine samples from 279 hospital outpatients and found
r 5 .88 between reagent strips and refractometry when no
correction was made for the presence of glucose in the urine
and r 5 .92 when a correction was made for glucose. They
suggested that the correction for glucose was appropriate but
probably academic. Finally, scientists from Miles Laboratories,13 which produces N-MULTISTIX reagent strips, compared their reagent strips with refractometry in 791 nonhospitalized subjects. They reported a correlation of .796 between
the reagent strips and refractometry and reported that the reagent strips were not affected by glucose in the urine, but that
urine protein may have had an effect. In contrast, other researchers12,16–19 assessing the validity of reagent strips with
refractometry have reported that the reagent strips are not an
acceptable alternative to refractometry. McCrossin and Roy12
and Adams16 reported correlation coefficients of r 5 .82 and
r 5 .80, respectively, between reagent strips and refractometry
and concluded that the reagent strips were not a valid measure
of urine specific gravity. Similar conclusions were made by
Zack (r 5 .791)19 and Brandon (r 5 .7246)17 when comparing
reagent-strip and refractometer measurements. Finally, Dorizzi
and Caputo18 compared 2 different reagent strips with refractometry in 1725 urine samples from hospital inpatients and
outpatients and reported correlation coefficients of only r 5
.663 and r 5 .514. In addition to concluding that reagent strips
cannot replace refractometry, these researchers also reported
that urine glucose and protein had no effect on reagent-strip
specific-gravity measurements.
Investigators in the previously described validity studies all
compared hydrometry or reagent strips with refractometry using
interclass Pearson correlation coefficients. However, interclass
correlations may not be the appropriate statistical technique because the same variable is being correlated. It has been suggested that, when multiple tests are given for the same variable,
intraclass correlation coefficients should be used.20 Furthermore,
Bland and Altman21 have proposed that the best statistical approach when comparing measurement devices is the Bland-Altman plot. This approach plots the differences between 2 mea-
surement devices against the mean of the measurement devices.
If the measurements using the 2 devices are comparable, the
differences on the plot should be small and centered on zero.
Using a variety of statistical approaches (analysis of variance, intraclass correlation coefficients, Bland-Altman plots,
and the calculation of false-positive and false-negative readings), we found that neither hydrometry nor reagent strips was
a valid measure of urine specific gravity in collegiate wrestlers.
Hydrometer and reagent-strip measurements were significantly greater than refractometer measurements, a finding also
shown with the Bland-Altman plots (see Figures 1 and 2). In
both plots, data points occurred outside the 95% limits of
agreement, which potentially influenced the analysis-of-variance results (decreased methods effect sizes) and the intraclass
correlation coefficients (decreased correlation coefficients). Intraclass correlations between reagent strips or hydrometry and
refractometry ranged from low to moderate. Because urine
specific gravity determines a wrestler’s hydration status during
the weight-certification process, consideration of false-positive
and false-negative hydrometer and reagent-strip results takes
on added importance. False positives occurred with both the
hydrometer (28%) and reagent strips (15%). These wrestlers
were euhydrated, but the hydrometer or reagent-strip measurement indicated that they were dehydrated, negating establishment of a weight class. False negatives occurred to a lesser
extent with the hydrometer (2%) and reagent strips (9%).
These wrestlers were dehydrated, but the hydrometer or reagent-strip measurement indicated euhydration, allowing the
establishment of a weight class. Because this would be determined with the wrestler dehydrated, the weight class for that
wrestler would be set incorrectly low. This is a health risk for
the wrestler, which is the very situation the NCAA was trying
to prevent with the establishment of the new wrestling rules.
Although it is difficult to directly compare our results with the
findings of others because of different statistical approaches,
our overall conclusion that hydrometry and reagent strips are
not acceptable substitutes for refractometry is consistent with
the findings of other validity studies discussed previously.12,16–19
A limitation of our study is that none of the subjects was
severely dehydrated, so we did not assess the reliability and
validity of the 3 methods in this situation. As described previously, McCrossin and Roy12 found that the correlation between refractometry and hydrometry diminished in the high
range of urine specific-gravity measurements. Furthermore, we
did not determine if the urine samples contained glucose or
protein. Although some researchers have suggested that glucose15 or protein13 may affect urine specific-gravity measurements with reagent strips, others have concluded that reagentstrip measurements are not affected by the presence of
glucose13,18 or protein,18 as discussed previously.
In conclusion, our results suggest that the refractometer is
a reliable measure of urine specific gravity. In contrast, the
hydrometer and reagent strips were not reliable, nor were they
valid measures of urine specific gravity when compared with
refractometry. When the new wrestling rules were first introduced in 1998,4 the NCAA allowed the use of all 3 methods
to assess urine specific gravity. Subsequently, the use of reagent strips was eliminated.6 Our data support this decision
and further suggest that the use of the hydrometer also should
be eliminated. The refractometer is reliable, fast, accurate, and
technically easy to use and requires only a single drop of urine.
Therefore, we suggest that refractometry should be the
NCAA’s only choice for measuring a collegiate wrestler’s urine
specific gravity during the weight-certification process. Future
research is warranted to assess whether a urine specific-gravity
measurement of #1.020 as selected by the NCAA is an appropriate cut-off value to indicate euhydration.
ACKNOWLEDGMENTS
We thank Sharon Birch for her statistical expertise and Michael
Cantele, Joseph Donolli, and Kristin Petrovia for their assistance in
data collection.
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